Atherosclerosis and vascular thrombosis are a major cause of morbidity and mortality, leading to coronary artery disease, myocardial infarction, and stroke. Atherosclerosis begins with an alteration in the endothelium, which lines the blood vessels. The endothelial alteration results in adherence of monocytes, which penetrate the endothelial lining and take up residence in the subintimal space between the endothelium and the vascular smooth muscle of the blood vessels. The monocytes absorb increasing amounts of cholesterol (largely in the form of oxidized or modified low-density lipoprotein) to form foam cells. Oxidized low-density lipoprotein (LDL) cholesterol alters the endothelium, and the underlying foam cells distort and eventually may even rupture through the endothelium.
Platelets adhere to the area of endothelial disruption and release a number of growth factors, including platelet derived growth factor (PDGF). PDGF, which is also released by foam cells and altered endothelial cells, stimulates migration and proliferation of vascular smooth muscle cells into the lesion. These smooth muscle cells release extracellular matrix (collagen and elastin) and the lesion continues to expand. Macrophages in the lesion elaborate proteases, and the resulting cell damage creates a necrotic core filled with cellular debris and lipid. The lesion is then referred to as a "complex lesion." Rupture of this lesion can lead to thrombosis and occlusion of the blood vessel. In the case of a coronary artery, rupture of a complex lesion may precipitate a myocardial infarction, whereas in the case of a carotid artery, stroke may ensue.
One of the treatments that cardiologists and other interventionalists employ to reopen a blood vessel which is narrowed by plaque is balloon angioplasty (approximately 300,000 coronary and 100,000 peripheral angioplasties are performed annually). Although balloon angioplasty is successful in a high percentage of the cases in opening the vessel, it unfortunately denudes the endothelium and injures the vessel in the process. This damage causes the migration and proliferation of vascular smooth muscle cells of the blood vessel into the area of injury to form a lesion, known as myointimal hyperplasia or restenosis. This new lesion leads to a recurrence of symptoms within three to six months after the angioplasty in a significant proportion of patients (30-40%).
Because of their great prevalence and serious consequences, it is critically important to find therapies which can diminish the incidence of atherosclerosis, vascular thrombosis and restenosis. Ideally, such therapies would inhibit the pathological processes associated with atherosclerosis, thereby providing prophylaxis or retarding the progression of the degenerative process.
As briefly summarized above, these pathological processes are extremely complex, involving a variety of different cells which undergo changes in their character, composition, and activity, as well as in the nature of the factors which they secrete and the receptors that are up- or down-regulated. A substance released by the endothelium, "endothelium derived relaxing factor" (EDRF), may play an important role in inhibiting these pathologic processes EDRF is now known to be nitric oxide (NO) or a labile nitroso compound which liberates NO. (For purposes of the subject invention, unless otherwise indicated, nitric oxide (NO) shall intend nitric oxide or the labile precursor.) This substance has been reported to relax vascular smooth muscle, inhibit platelet aggregations, inhibit mitogenesis and proliferation of cultured vascular smooth muscle, and leukocyte adherence. NO may have other effects, either direct or indirect, on the various cells associated with vascular walls and degenerative diseases of the vessel.
Relevant Literature
Girerd, et al. (1990) Circulation Research 67, 1301-1308 report that intravenous administration of L-arginine potentiates endothelium-dependent relaxation in the hind limb of cholesterol-fed rabbits. The authors conclude that synthesis of EDRF can be increased by L-arginine in hypercholesterolemia. Rossitch, et al. (1991) J. Clin. Invst. 87, 1295-1299 report that in vitro administration of L-arginine to basilar arteries of hypercholesterolemic rabbits reverses the impairment of endothelium-dependent vasodilation and reduces vasoconstriction. They conclude that the abnormal vascular responses in hypercholesterolemic animals is due to a reversible reduction in intracellular arginine availability for metabolism to nitric oxide.
Creager, et al. (1992) J. Clin. Invest. 90, 1248-1253, report that intravenous administration of L-arginine improves endothelium-derived NO-dependent vasodilation in hypercholesterolemic patients.
Cooke, et al., "Endothelial Dysfunction in Hypercholesterolemia is Corrected by L-arginine," Endothelial Mechanisms of Vasomotor Control, eds. Drexler, Zeiher, Bassenge, and Just; Steinkopff Verlag Darmstadt, 1991, pp. 173-181, review the results of the earlier references and suggest, "If the result of these investigations may be extrapolated, exogenous administration of L-arginine (i.e., in the form of dietary supplements) might represent a therapeutic adjunct in the treatment and/or prevention of atherosclerosis."
Cooke, (1990) Current Opinion in Cardiology 5, 637-644 discusses the role of the endothelium in the atherosclerosis and restenosis, and the effect that these disorders have on endothelial function.
Cooke (1992) J. Clin. Invest. 90, 1168-1172, describe the effect of chronic administration of oral L-arginine in hypercholesterolemic animals on atherosclerosis. This is the first demonstration that oral L-arginine supplements can improve the release of NO from the vessel wall. The increase in NO release from the vessel wall was associated with a striking reduction in atherosclerosis in hypercholesterolemic animals. This is the first evidence to support the hypothesis that increasing NO production by the vessel wall inhibits the development of atherosclerosis.
Cooke and Tsao, (1992) Current Opinion in Cardiology 7, 799-804 describe the mechanism of the progression of atherosclerosis and suggest that inhibition of nitric oxide may disturb vascular homeostasis and contribute to atherogenesis.
Cooke and Santosa, (1993) In: Steroid Hormones and Dysfunctional Bleeding, AAAS Press, review the activities of EDRF in a variety of roles and suggest that reversibility of endothelial dysfunction may be affected by the stage of atherosclerosis. They conclude that EDRF is a potent vasodilator, plays a key role in modulating conduit and resistance vessel tone, has important effects on cell growth and interactions of circulatory blood cells with a vessel wall, and that disturbances of EDRF activity may initiate or contribute to septic shock, hypertension, vasospasm, toxemia and atherosclerosis.
Other references which refer to activities attributed to NO or its precursor include: Pohl and Busse (1989) Circ. Res. 65:1798-1803; Radomski et al. (1987) Br. J. Pharmacol. 92:181-1187; and Stamler et al. (1989) Circ. Res. 65:789-795; anti-platelet activity); Garg and Hassid (1989) J. Clin. Invest. 83:1774-1777; and Weidinger et al, (1990) Circulation 81:1667-1679; NO activity in relation to vascular smooth muscle growth); Ross (1986) N. Engl. J. Med. 314:488-500; Bath et al. (1991) Arterioscler. Thromb. 11:254-260; Kubes et al. (1991) Proc. Natl. Acad. Sci. USA 89:6348-6352; Lefer et al. (1990) In: Endothelium-Derived Contracting Factors. Basel, S. Karger, pp. 190-197; NO activity in relation to leukocyte adhesion and migration); Heistad et al. (1984) Circ. Res. 43:711-718; Rossitch et al. (1991) J. Clin. Invest. 87:1295-1299; Yamamoto et al. (1988)ibid 81:1752-1758; Andrews et al. (1987) Nature 327:237-239; Tomita et al. (1990)Circ. Res. 66:18-27; Kugiyama et al. (1990) Nature 344:160-162; Mitchell et al. (1992) J. Vasc. Res. 29:169 (abst.); and Minor et al. (1990) J. Clin. Invest. 86:2109-2116; NO activity in relation to hypercholesterolemia); Tanner et al. (1991) Circulation 83:2012-2020; Kuo et al. (1992) Circ. Res. 70:f465-476; Drexler et al. (1991) Lancet 338:1546-1550; and Nakanishi et al. (1992) In: Scientific Conference on Functional and Structural Mechanisms of Vascular Control, Snowbird, UT, p. 86 (abstsr.); relation of L-arginine to NO-dependent vasodilation.